Gold tin alloy clad product



Feb. 6, 1968 J, c, LA -r5 3,367,756 GOLD TIN ALLOY GLAD PRODUCT FiledMarch 25, 1966 V INVENTOR. JERRY C.LA PLANTE BYXW United States Patent 1Claim. '(Cl. 29-199 This application is a continuation in part of mycopending application entitled Gold Alloy Clad Products, Serial No.428,750, filed January 28, 1965, now Patent 3,245,764.

This invention relates to means and methods of cladding gold tin alloyson substrates such as nickel, nickel iron, Kovar, molybdenum and relatedmaterials.

More specifically, we are referring to gold tin alloys of 15% to 25%tin, the balance being gold.

It is difficult to achieve good cladding due to the fact that the goldalloys do not normally form a good bond with the substrate material oreven with a gold clad substrate material.

After many unsuccessful experiments, good results were achieved by firstleaching the gold alloy to form a porous surface layer of gold. Thisporous layer then will form a good bond with a gold clad substrate, forinstance by hot rolling.

Accordingly, a principal object of the invention is to provide new andimproved products of clad gold tin alloys on a substrate metal.

Another object of the invention is to provide a new and improved productcomprising a leached gold tin alloy clad on a metal substrate.

Another object of the invention is to provide new and improved productsof clad gold tin alloys made by the process of leaching the gold alloysto form a porous gold layer and then bonding said porous layer onto agold clad substrate material.

Another object of the invention is to provide new and improved productsof clad gold tin alloys on metals such as nickel, nickel iron, Kovar,molybdenum, and related materials.

Another object of the invention is to provide new and improved productsof leached clad gold tin foils.

These and other objects of the invention will become apparent from thefollowing specification and drawing which is a cross sectional view of agold tin silicon alloy clad on a metal substrate.

The following unsuccessful experiments were made:

Example I.--Alloy the gold tin eutectic directly onto the substrate bypuddling or melting, using a variety of fluxing and reducing conditions.Due to the large thermal expansion difference between the gold eutecticsand the substrate materials, no adequate bond was created with aresulting cracking and peeling at the interface.

Example 2.A thin alloy gold cladding was put on the substrate and thenthe gold tin alloy was alloyed directly onto the clad surface. This,too, proved unsuccessful for the same reasons.

Example 3.-Pressure bonding directly on the substrate was attempted.Results again were unsuccessful.

Example 4.Pressure bonding plus heat were employed, directly on thesubstrate. This, too, proved unsuccessful.

Example 5.Pressure bonding plus heat on the thin gold clad substrate wasattempted. Results were again similar.

It became obvious that a bond must be made at a temperature low enoughto prevent peeling due to expansion difference. This would entail solidstate bonding. Conventional techniques of pressure bonding to the basemetal were unsuccessful. Further work with pressure bonding (plus heat)onto pure gold preclad material was only partially successful.

It became apparent that bonding was being prevented by the existence oftin rich eutectic phase-present on the eutectic alloy surface.

This was eliminated by leaching of the alloy material to leach out thetin rich phase from the surface by immersion in a proper leachingsolution for an appropriate length of time. For example, if the tinalloy is leached for six hours in concentrated HF the surface isdepleted of the tin phase to a depth of approximately .0001 inch. Thisproduces a pure gold surface which is readily bonded by standard heatand/or pressure techniques to a gold preclad base material.

The general successful process is as follows:

The gold tin alloy is leached for six hours in concentrated hydrofluoricacid to deplete the surface of the tin in the alloy to a depth ofapproximately A of a mil, thereby providing a gold rich, probably porouslayer.

The alloys referred to are gold tin 15% to 25 tin.

The alloy sheet is then bonded to a gold clad substrate by hot rolling,at a temperature below the melting point of the alloy.

In one specific embodiment it was desired to clad gold tin on Kovarhaving a gold layer on the other side of the Kovar. The gold layer onthe other side of the Kovar is not necesary to the present process. Thefollowing steps were taken:

(1) Take a sheet of gold foil .070" thick having convenient dimensionsfor instance 3" x 6".

(2) An ingot of substrate Kovar approximately A" thick having equaldimensions.

(3) A sheet of gold foil .005".00l" thick having equal dimensions.

(4) Sandwich the Kovar between the gold sheets and Weld around the edgeswith a heliarc torch.

(5) Heat the assembly to approximately 1300 F. in a reducing atmosphere,for instance of hydrogen.

(6) Roll out the assembly in a rolling mill to approximately .075"thick. The thickness is reduced approximately 25 per pass in the rollingmill.

(7) Anneal at a temperature of 13001400 F. for approximately 1 hour.

(8) Cool in a reducing atmosphere.

(9) To apply the gold tin take a sheet of gold tin foil approximately.015 thick of the same size as the rolled out assembly.

The optimum composition is Au/20% Sn. The 80% Au composition is wheneutectic occurs in the Au/ Sn system. This composition has a meltingpoint of 280 C. and as such is a low melting Au solder. The solder hasthe general characteristics of being very resistant to low acid etchesand is, therefore, far superior to high Sn compositions forsemiconductor applications. It also has the prop erties of beingconsiderably stronger than the high Sn compositions as well as harderthan the high Au compositions.

(10) Leach the gold tin foil in hydrofluoric acid or equivalent forapproximately six hours. This may be at room temperature as thetemperature is not critical. The leaching process removes the tin alongthe surface leaving a porous surface layer of gold.

(11) The leached gold tin foil is then placed on the thin gold clad sideof the Kovar of the three-layer assembly. This four-layer assembly isthen rolled at approximately 500 down to approximately .009". The poroussurface of gold on the alloy forms a good bond with the gold clad Kovar.

In the above example it was desired to sandwich the Kovar between thealloy and a second layer of gold. If the second gold layer is notdesired, it may be merely eliminated from the process. It does notaffect the bonding of the alloy.

The figure shows a cross sectional view of a gold tin sheet of foil 1having a leached surface clad onto a metal substrate 2 for instance, ofthe group including nickel, nickel iron, Kovar, molybdenum or otherequivalent materials, according to the process described.

Many modifications may be made by those who desire to practice theinvention without departing from the scope thereof which is defined bythe following claim.

I claim:

1. The product consisting of a metal substrate, a gold layermechanically clad on said substrate, and a gold tin alloy mechanicallyclad on said gold layer and thus bonded to the metal substrate, saidgold tin alloy consisting of 15% to 25% tin.

References Cited UNITED STATES PATENTS 440,693 11/1890 Burden 291991,008,970 11/ 1911 Kessler 29199 1,090,939 3/1914 Newton 29199 3,031,7475/1962 Green 39-199 3,245,764 4/1966 La Plante 29-196 FOREIGN PATENTS24,044- 1911 Great Britain.

OTHER REFERENCES Constitution of Binny Alloys by Dr. Hansen, published1958 by McGraw-Hill Book C0,, pp. 232234.

HYLAND BIZOT, Primary Examiner

1. THE PRODUCT CONSISTING OF A METAL SUBSTRATE, A GOLD LAYERMECHANICALLY CLAD ON SAID SUBSTRATE, AND A GOLD IN ALLOY MECHANICALLYCLAD ON SAID GOLD LAYER AND THUS BONDED TO THE METAL SUBSTRATE, SAIDGOLD TIN ALLOY CONSISTING OF 15% TO 25% TIN.